Hey all, While trying to get a commit pushed for Yesod[1], Alexander Dunlap pointed out one of his programs didn't work with the new code. After some investigation, I was able to reproduce the bug with the following code snippet: {-# LANGUAGE TypeFamilies #-} data family Foo a data Bar = Bar data instance Foo Bar = Bar1 | Bar2 | Bar3 | Bar4 | Bar5 | Bar6 | Bar7 | Bar8 | Bar9 deriving Eq This produces: Couldn't match expected type `Main.R:FooBar' against inferred type `Foo Bar' NB: `Foo' is a type function In the first argument of `Main.$con2tag_R:FooBar', namely `a' In the expression: (Main.$con2tag_R:FooBar a) In the expression: case (Main.$con2tag_R:FooBar a) of { a# -> case (Main.$con2tag_R:FooBar b) of { b# -> (a# GHC.Prim.==# b#) } } The especially strange thing about this bug is that it only occurs when there are more than 8 constructors; if I remove Bar9, everything seems to work. Does anyone have experience with this occuring? Michael [1] http://docs.yesodweb.com/blog/please-break-yesod/